1. Field of the Invention
The present invention relates to a medical scanning system and a related method of scanning. More particularly, this invention relates to a medical scanning system and a related method that uses uniform rotary motion of an optical reflector to create reciprocal linear scanning.
2. Background of the Related Art
Various types of medical imaging techniques are currently used for diagnosis and treatment of a patient. These imaging techniques include ultrasound imaging and Optical Coherence Tomography (OCT). Ultrasound uses sound waves to obtain a cross-sectional image. These waves are radiated by a transducer and directed into the tissues of a patient. Waves reflected from tissues at different depths excite the same transducer, which now acts as a receiver. The transducer converts the reflected waves into electrical signals, which are electronically processed and ultimately displayed. The typical tissue depth applicable to ultrasound imaging varies, depending on the application, from millimeters to centimeters.
OCT uses electrical light to obtain a cross-sectional image of tissue. Since light waves are faster than sound waves, OCT takes a different approach to imaging. The depth of tissue scan in OCT is based on low coherence interferometry. Low coherence interferometry measures the field of an optical beam rather than its intensity. It preferably uses an interferometer with reference arm scanning and a low coherence light source. In use, a low coherence light source of the interferometer is directed onto a beam splitter to produce two beams, a sampling measurement beam and a reference beam. The sampling beam hits and penetrates the tissue or material to be imaged, and then reflects (backscatters) from the tissue, carrying information about the reflecting points from the surface and the depth of the tissue. The reference beam hits a reference reflector, for example a mirror or a diffraction grating, and reflects from the reference reflector. The reference beam travels a given path length, such that the reference reflector either moves or is designed such that the reflection occurs at different distances from the beam splitting point and returns at a different point in time or in space, which actually represents the depth scanning. The amount of such movement represents the desirable depth of penetration of the tissue or object being imaged by the sampling beam. Typical such depths in OCT are 2 to 3 millimeters.
The output of the interferometer is the superposition of the electromagnetic fields from the reflected reference beam and the sampling beam reflected from the tissue or material being imaged. When the reflected beams meet, interference is observed only where the path lengths of the reference arm and sampling arm are matched to within the coherence length of the light. A photodetector detects this interference and converts it into electrical signals. The signals are electronically processed and ultimately displayed, for example, on a computer screen or other monitor.
Each cross-sectional image involves two scans: depth (axial) and width (lateral). Typically, the rate of depth scan is faster than the rate of lateral scan, as 200 to 300 or more depth scans may occur for one lateral scan during live imaging. A typical rate of lateral scanning during live imaging is approximately 26-30 scans per second.
A typical OCT probe for linear cross sectional imaging uses a mechanical scanning arrangement in which at least one mechanical part reciprocates to create a scanning motion. The reciprocal motion, however, creates drawbacks associated with the inertia of moving parts. These drawbacks affect the accuracy of the scan. For example, reciprocal motion involves nonuniform speed of scan, i.e. the scanning speed decreases to zero at the end of each reciprocating cycle. In addition, vibration of the reciprocating mechanical parts results in an unstable scanning system and an elevated level of electronic noise. These problems increase as the scanning speed increases during live imaging.